The present invention relates to digital radio systems, and more specifically, to the determination of long code groups as part of the processing of a received signal in a spread spectrum radiocommunication system.
Radiocommunication systems involve the transmission of information over an air interface, for example by modulating a carrier frequency with that information. Upon reception, a receiver attempts to accurately extract the information from the received signal by performing an appropriate demodulation technique. Some systems provide channelization using a spread spectrum technique known as code division multiple access (CDMA). In some CDMA systems, the information data stream to be transmitted can first be coded or spread using a unique spreading code and then combined with a long PN-sequence or a shorter scrambling-sequence, collectively referred to herein as xe2x80x9clong codesxe2x80x9d. The long codes can be planned from cell to cell so that neighboring cells use different long codes. The information data stream and the long code can have the same or different bit rates. The multiplication of the information data stream with the unique spreading code and long code results in an output stream of chips.
To further understand the usage of long codes associated with signal processing in a CDMA radiocommunication system, consider the following example. FIG. 1 illustrates the use of base stations to transmit radio waves to mobile users (mobile stations) in a cellular system. In a CDMA system, base station 10 can transmit signals to mobile stations 14 and 15 as a single (composite) signal. The signal directed to mobile station 14 is typically coded with a short code that is orthogonal or mostly orthogonal to a short code that is used to code the signal directed to mobile station 15. These signals are then scrambled with a second code that is sometimes referred to as a long code, associated with base station 10. The sum of the two coded and spread signals is then transmitted by base station 10.
When mobile station 14 receives the composite signal, mobile station 14 multiplies the spread signal with the long code and the short code to recreate the signal directed to mobile station 14 and the signal directed to mobile station 15 is suppressed as interference noise. Similarly, mobile station 15 multiplies the spread signal with the long code and the short code assigned to mobile station 15 to recreate the signal directed to mobile station 15 and the signal directed to mobile station 14 is suppressed as interference noise. To perform this processing on the received signals mobile stations 14 and 15 must have identified the long code used to scramble the received signal, in addition to learning or knowing the applicable short codes and having attained time synchronization.
In an exemplary CDMA system, the mobile stations are able to identify the long code used by a particular base station by listening to a control channel known as the synchronization channel (SCH). Decoding of the SCH is performed during both an initial xe2x80x9cpower-onxe2x80x9d synchronization phase and when measurements of neighboring base stations"" SCH are performed for cell reselection. One symbol is transmitted in the SCH during every slot. The SCH consists of two subchannels, the Primary SCH (P-SCH) and the Secondary SCH (S-SCH), which are transmitted in parallel from the base station. The P-SCH always carries the same symbol. A mobile station listens to the P-SCH to detect the timing of the symbols of the S-SCH, and thereby the slot timing. After the P-SCH is detected the S-SCH is read using the P-SCH as a phase reference.
However, when in initial synchronization a mobile station has no knowledge of which particular long code in the set of all available long codes might be received. Moreover, during the initial synchronization phase, frame synchronization to the S-SCH has not yet been achieved and therefore, the mobile station has no knowledge when the first symbol in a code sequence is received. The decoder must therefore take into account that all time shifts of each available long codeword can be received as part of its attempt to identify the long code needed to scramble a particular SCH""s transmission.
As mentioned above, mobile stations 14 and 15 may be performing measurements on signals from other base stations in surrounding cells, i.e., base stations 20, 30 and 40, while communicating with base station 10 to ensure that they are currently communicating with the best base station(s). In order to perform the measurements it is necessary to determine the long codes used by the various base stations. When mobile stations 14 and 15 are in the measurement mode they may receive a list of neighboring base stations and which long code group they are using. Hence, the search for specific codewords becomes limited, e.g., identifying one codeword out of a group of sixteen.
It is desirable to perform the determination of the long codes as quickly as possible. For example, when an interfrequency handover is prepared, measurements of base stations on other frequencies must be performed. Therefore, the time for each measurement should be minimized because the normal traffic to the terminal is interrupted when measurements are performed on other frequencies. Additionally, since mobile stations typically use batteries as a source of power, it is desirable to perform the decoding quickly, in order to use less power. Further, by performing the decoding quickly, the required amount of hardware is minimized because the measurement hardware can be time shared.
In conventional systems, a correlation for all 16 symbols are performed for the 15 symbols over the frame. Then all long code group code sequences in any of the 15 phases are used for correlation with the received sequence and the sequence with the highest correlation value is selected as being the correct long code group. Performing the correlations according to this conventional method requires a complex decoder because of the many operations which are required. For example, if there are 32 long code groups, then the number of sequences which are used as candidates are 15*32=480. Thus, the correlations for 480 sequences must be calculated and the best one shall be selected. Another drawback of this conventional method is that it requires a considerable amount of buffering to perform the required correlations. Further, if the number of long code groups are increased, the decoding complexity is substantially increased. Accordingly, a method and apparatus for performing long code group detection using less than the total number of symbols in the codewords would be desirable.
The present invention relates to reducing the number of bits or symbols which need to be evaluated in order to determine the particular long code group associated with the particular long code used to scramble transmissions by a particular base station. According to exemplary embodiments of the present invention, a broadcast control channel includes a synchronization bit or symbol in each of a plurality of time slots. The sequential nature of these bits or symbols can be used by the receiver to determine which long code group corresponds to the received code sequence.
According to one exemplary embodiment of the present invention, a table of code sequences is built. The received symbols are compared to the table and a determination is made as to the detected long code group. In an alternate exemplary embodiment of the present invention, an iterative process is used to determine a sequence of symbols which identify a particular long code group through the use of hard or soft metrics.
Since the present invention does not require the decoding and correlation of all of the symbols of a long code group to uniquely identify the long code group, the complexity of decoding is decreased due to the reduced amount memory required for buffering the received code sequence. Further, the complexity of decoding is decreased since it is not necessary to determine the metric of all of the symbols in the received code sequence.